In an image-forming process by electrophotographic system, a visible image is obtained by forming an electrostatic latent image on a photosensitive material comprising an inorganic or organic material, developing the electrostatic latent image with a toner, transferring the developed image onto a paper, a plastic film or the like, and fixing the transferred image thereon. The photosensitive material has a positive chargeability or a negative chargeability depending on its constitution, and when leaving an electrostatic image on a part to be printed by light exposure, development is carried out with a reversely charged toner. On the other hand, when carrying out reverse development by destaticizing a part to be printed, development is carried out with the same side charged toner.
A toner comprises a binding resin, a coloring agent and other additives. A charge controlling agent is generally added in order to provide satisfactory tribo-chargeabilities (including a charging speed, a charging level, a charging stability or the like), a desirable stability as a lapse of time and a satisfactory environmental stability. Properties of the toner are largely influenced by addition of the charge controlling agent.
In case of a color toner, the market of which is expected to become large in future, it is indispensable to use a pale color, preferably colorless charge controlling agent which does not provide an influence on hue. Examples of a conventional charge controlling agent include a metal complex salt compound of a salicylic acid derivative (JP-B-55-42752, JP-A-61-69073, JP-A-61-221756, JP-A-9-124659 and the like), an aromatic dicarboxylic acid metal salt compound (JP-A-57-111541 and the like), a metal complex salt compound of an anthranilic acid derivative (JP-A-62-94856 and the like), and an organic boron compound (U.S. Pat. No. 4,767,688, JP-A-1-306861 and the like).
However, these charge controlling agents have disadvantages that some of them are chromium compounds hardly usable due to more strictly required environmental safety, that some of them are not colorless or do not have a satisfactory pale color required for a color toner, and that some of them are poor in a charge-imparting effect, a chargeability of a toner, or a dispersibility or stability of a compound. JP-A-62-163061 discloses an effective photographic toner containing a calcium salt of 3,5-di-tert-butylsalicylic acid. A charge controlling agent comprising a calcium salt of 3,5-di-tert-butylsalicylic acid disclosed in this publication has a pale color and do not contain a heavy metal such as chromium, and is therefore usable for a color toner. However, although this charge controlling agent is considered not to contain a heavy metal such as chromium, it does not achieve a satisfactory charge-imparting effect demanded nowadays and has a defect of being liable to cause image degradation during long term running. Thus, it is demanded to provide a charge controlling agent having a high charge-imparting effect.